Method and apparatus for the fabrication of electrostatic recording heads



March 14, 1961 c. R. JOYCE 2,974,368

METHOD AND APPARATUS FOR THE FABRICATION OF ELECTROSTATIC RECORDING HEADS 2 Sheets-Sheet 1 #1 23 ANVIL ELECTROSTATIC COUNTER Filed Sept. 12, 1956 GAS AND WATE VAPOR JET g;

ADJUSTABLE HEAD MOUNTING FIXTURE HEAD March 14, 1961 c. R. JOYCE 2,974,368

METHOD AND APPARATUS FOR THE FABRICATION OF ELECTROSTATIC RECORDING HEADS 2 Sheets-Sheet 2 Filed Sept. 12, 1956 INVENTOR. CECIL RI JOYCE AGENT United States Patent METHOD AND APPARATUS FOR THE FABRICA- TION OF ELECTROSTATIC RECORDING HEADS Cecil R. Joyce, Paoli, Pa., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Filed Sept. 12, 1956, Ser. No. 609,431

4 Claims. (Cl. 18-36) The present invention relates to electrostatic recording and more particularly to a novel head construction having an array of electrodes for printing or plotting data upon a suitable responsive medium, and to the method of and apparatus for fabricating the same.

In the development of electrostatic printing, effective results largely depend upon complicated circuits for character selection requiring a multiplicity of printing electrodes. In the past, and particularly where commercial production is concerned, the problem of properly arranging, separating and mounting such electrodes has introduced a prohibitive cost factor which has seriously retarded efforts to meet trade demands for such equipment.

An important object of the present invention is to provide a novel inexpensive and rapid method of fabricating an electrostatic printing head.

In accordance with the objects of the invention and first briefly described, there is provided a novel method and apparatus for the fabrication of electrostatic printing heads, wherein a plurality of conducting wires are arranged in spaced relation through a molded body of dielectric material with the ends of the Wires exposed thus to form a linear array of electrodes, each electrode of which is connected at its other end to a terminal connector. The head is fabricated by first securing the wires under tension, and the terminals in a mold and then layer molding insulating material around the wires and terminals in a manner whereby the molded material is free of bubbles and internal stresses.

In the accompanying drawings:

Figure 1 is an elevational view of a portion of apparatus employing an electrostatic head embodying one form of the present invention;

Figure 2 is a plan view of a wire holding element used in manufacturing the present invention;

Figure 3 is a side view, partially in phantom outline, .of the head of Figure 1;

Figure 4 is a view on an enlarged scale of a frag- :mentary portion of the head structure looking in the direction of the arrows on line 44 of Figure 3;

Figure 5 is a perspective view of one form of mold and wire tensioning mechanism for carrying out the method of the invention;

Figure 6 is an exploded perspective view of the head :and an associated electrical connector;

Figure 7 is a front view of the recording head of the present invention and showing an associated recording medium in phantom outline; and,

1 molded of a chemically exothermic material such as epoxy resin. The opposite end panel of body 10' has embedded therein, connector units 13 and 14, each of which, as seen in Figures 1 and 3, includes a plurality of rows of terminals 15 extending therethrough. The inner ends of the terminals are respectively soldered to a plurality of conducting wires 16, while the outer ends thereof project as plug-in connectors, engageable with the wires of a cable pulsing unit or the like, such as seen in Figures 1 and 6. Illustratively as seen in Figures 2 and 3, each connector unit is provided with three rows of terminals 15 with a wire 16, connected at one end to and leading from each terminal, and its other end terminating as a pin-shaped recording electrode 17 projecting from face 12 in a linear array, as indicated in Figure 7.

In the present instance, connector units 13 and 14 are arranged one above the other (Figures 2 and 3), with all the wires 16 meeting in a common row formed by passing them over a dowel 20 in spaced parallel relation with each other throughout the length of body extension 11. Thus the molded assembly includes in one portion thereof, six rows of fanned-out wires 16 (Figure 3) each row being in a transverse plane angularly spaced from each other, and all concentrated at another portion of the assembly in a single planar row. According to the present illustrated embodiment, even though eightytwo terminals are provided by connector units 13 and 14 only seventy-seven wires are used to form the linear array of exposed electrodes 17, some of which are shown in Figure 7.

Further, for a clear understanding of the critical nature of the assembly, it is to be understood that each wire has a diameter of the order of 0.003 of an inch. Proper parallel spacing of the wires is obtained by forming accurately spaced wire seating grooves in the dowel 20, as described later during the description of the method of fabrication.

Referring to Figure 1, illustrating a use of the head for electrostatic printing or the like, the recording head of the invention is arranged with its linear array of electrodes adjacent to a recording medium 21, in tape or sheet form, which is moved across the electrodes by means (not shown), but during which movement it is guided by suitable rollers 22 and 23 located on the 0p posits sides of a printing anvil 24 in such a way that the sheet 21 passes the electrode array in a direction normal to the alignment of the electrode array. Illustratively, spacing of the electrodes from medium 21 is of the order of a few thousandths of an inch. Fora more complete description of electrostatic printing apparatus wherein the present invention may be utilized, reference may be had to co-pending United States application, Serial Number 443,646, filed July 15, 1954, in the names of H. Epstein and F. Innes.

For performing the steps of the method of fabrication resulting in the complete head, there is provided an open top receptacle or mold 25, formed, as shown most clearly in Figure 5, by removable side walls 26 preferably of metal stock, a floor 27 shaped to produce the desired molded head contour, and an end panel 28 of dielectric material, the latter being cut from a larger piece 28a of such material as will be explained presently. The floor 27 provides a horizontal surface 27a and downwardly inclined surfaces 29 and 29a at opposite sides thereof. Dowel 20, of dielectric material having its ends fixed in walls 26, is positioned in the mold at the junction of walls 27a and 29. Mounted on inclined wall 29a as by angle irons 30, is a plate or platform 31 having upstanding rigid pins 32 which serve as anchoring means during tensioning of the wires. The bottom of dowel 20 rests in a transverse arcuately shaped groove in the receptacle floor at the junction of the two surfaces 27a and 29, and

' arranged across the outer end of floor 27a.

can be passed leaving a clearance.

is provided with wire spacing means comprising a row of peripheral grooves 33, Figures 4 and 5, each receiving one of the wires. V-grooves enable use of different wire diameters.

In order to facilitate handling and alignment of the multiplicity of fine wires 16 in an orderly array prior to their attachment to the molding fixture, connectors 13 and 14 are inserted and secured in suitable fashion, as by glueing, in apertures 13' and 14' in a thin sheet of dielectric material 28a (Figure 2). Individual wires 16, are knotted at one end and thereafter soldered to one of the connectorterminal pins 15, after which the opposite ends are secured in orderly array in the numbered slits 17 across the top of the dielectric sheet.

Sheet 28a is then placed in the jig as shown fragmentarily by the phantom outline 28a in Figure 5. Thereafter individual wires are removed from the slits and secured to one of the pins 32, by means of a spring 37, after which the excess material of sheet 28:: is cut away leaving panel 28.

As seen in Figure 5, when panel 28 is in position, terminals 15 are below dowel 20 and preferably below a plane parallel with and above floor 27a. In stringing the wires, each is brought upwardly and over dowel 20 across floor 27a and over a second dowel 34 transversely Preferably dowel '34 is formed with an outer brass sleeve having a row of peripheral grooves 35, equal in number to those in dowel 20 and aligned therewith to maintain uniform spacing of the parallel wires. Illustratively, the axial spacing of both sets of grooves 33 and 35 may be of the order of twenty thousandths of an inch center to center.

To tension wires 16 (only one being shown in Figure 5), each is led from dowel 34, downwardly over floor 29a, platform 31 and is connected by means of a clip 36 to a coil tensioning spring 37 attached to one of the pins 32. A selected applied tension by each of the springs 37 is of the order of one pound.

An inverted channel-shaped member 38, shown most clearly in Figure 8, extends across the mold between walls 26 in close proximity to dowel 34. A pair of U-shaped members 3333 adapted to be fitted over the upstanding projections 39-39 of the member 38 thus provide a slot or aperture through which the wires 16 The member 38 extends transversely of said mold and parallel to the dowel '34 at the required level to permit all of the tensioned wires 16 to pass through an exposed body of grease 40, such as Dow Corning silicone grease which fills the member 38 and together with the members 3333 forms a dam at this end of the mold to prevent the molding material from running out.

After all of the wires are attached to pins 32, the aforementioned liquid resinous material is poured into the mold cavity formed by the floor 27, and walls 26, such pouring being a step-by-step operation starting in the area of the inclined floor 29 thus to form a composite body of successive layers. As each layer is poured to its selected thickness, it is allowed to cool slightly before adding a succeeding layer. Thus, layer by layer the body grows, passing over dowel 20 to flow onto the horizontal floor surface 27a until approximately one-sixteenth of an inch below the plane of the stretched wires 16. With the cooling of this layer, pouring continues until the material reaches a level approximately above the stretched wires, and then it is allowed to cool slightly.

This cooled layer is now covered with another layer of the plane of wire prevent the formation of bubbles about the wires, and thebuild-up ofinternal stresses as well as 4 reducing shrinkage of the plastic molding material. Electrical arc-through in the bubbles due to gas formation therein would tend to cause carbon tracks in the plastic material shorting the wires 16 together.

When the molding operation is complete, the mold is dismantled thus to release the molded head, after which the extension is cut off transversely in close proximity to dam 38, thereby exposing the array of electrodes for final processing as exposed pointed pin members. Also as seen in Figs. 3 and 4 dowel 20 is trimmed down to conform to the molded contour of the body of the head.

The wire ends are or may be abraded in suitable fashion as with a pig bristle brush to remove the excess plastic material adhering thereto since the plastic tends to expand beyond the tips of the exposed wires.

It will now be apparent that a novel method and apparatus for making a complete unitary electrostatic printing head has been devised, resulting in a head construction wherein a multiplicity of pre-stressed accurately spaced leads are so arranged as to form a single linear array of exposed electrodes.

What is claimed is:

l. A method of forming an electrostatic printing head, comprising: providing a mold formed by separable members including, a bottom and side walls; bridging said side walls in one end of the mold with a first rigid member; bridging said side Walls intermediate the ends of said mold with a second rigid member formed of dielectric material; providing a dam at said one end of said.

mold between said members; providing an opening in said dam through which wires may be passed; sealing said opening with a body of relatively viscous self-supporting substance; attaching one end of each of a plurality of line conductive Wires sequentially and in spaced alignment across a mold member damming the other end of said mold and at a level below a plane across the top surface of said first and second rigid members; stringing said wires in spaced alignment upwardly across and in contact with the top surface of said second rigid member, through said opening in said dam and through said viscous self-supporting substance, over and in contact with the upper surface of said first rigid member while maintaining said wires under tension by their other ends; pouring thermosetting insulating material into said mold intermittently in layers to successive levels until the wires are covered, allowing each pouring to partially set and to cool before pouring the succeeding layer, thus to form the printing head; separating said mold from said head, retaining said second rigid member as an integral part of said head; and finally cutting off said other ends of said wires.

2. A method of forming an electrostatic printing head, comprising: providing a mold formed by separable members including,-a bottom and side walls; bridging said side walls at one end of the mold with a first rigid member having peripheral grooves; bridging said side walls at a point spaced from said first bridging member with a second rigid member formed of dielectric material and having peripheral grooves aligned respectively with the grooves of said first rigid member; providing a dam at said one end of said mold between said rigid members and adjacent to said first rigid member; providing an opening in said dam through which wires may be passed; sealing the opening in said dam with a body of relatively viscous self-supporting substance, attaching one end of each of a plurality of fine conductive wires in spaced alignment to a plurality of electrical terminals disposed across a member damming the other end of said mold and at a level below a plane across and tangent with the top surfaces of said right members; stringing said wires upwardly and through the grooves in the surface of said second rigid member, through said opening in said dam through said viscous self-supporting substance, over said firstrigid member and through the grooves thereof, thus to maintain their spaced alignment; maintaining said wires in tension over said rigid members by means of their other ends; pouring thermosetting insulating material into said mold intermittently in thin layers to successive levels until the wires are covered, allowing each pouring to partially set and to cool before pouring the succeeding layer thus to form a molded head body; separating said mold from said head body, retaining said second rigid member and said dam member to which said wire ends are attached as integral parts thereof; and finally cutting 01f said other wire ends flush with said head body, thus to form an electrostatic printing head having an aligned row of printing electrodes exposed at one end, terminal connections at the other, and a reinforcing wire supporting member extending transversely through its molded body.

3. A mold fixture comprising: a rectangular troughlike member open at opposite ends and at the top, a member of dielectric material transversely spanning said trough-like member at one of said open ends, said member being provided with an opening into which an electrical terminal supporting member is receivable to cooperate With said dielectric member to form a closure for that end, tensioning means disposed at the opposite ends of said rectangular member, a first grooved strand positioning member disposed within and transversely across the other open end of said rectangular member, a second strand positioning member of dielectric material disposed adjacent and transversely across and intermediate the opposite ends of said rectangular member, and a body of electric relatively viscous self-supporting material positioned intermediate and parallel to said strand positioning members and forming a dam across said other open end of said rectangular member.

4. A fixture for molding an electrostatic transducer wherein electrically conductive wires are employed as recording styli comprising, a trough-like member open at the ends and top thereby providing a molding form, a member disposed transversely across one of the open ends of the molding form and secured to opposite sides of said trough-like member for damming that end, a plate disposed adjacent to the opposite open end of said molding form and projecting outwardly away therefrom, a plurality of pin-like projections disposed on and projecting from one side of said plate, a first grooved wire positioning member located within said molding form and extending transversely across the trough-like member, a second grooved wire positioning member located within said molding form spaced from the first positioning member and extending transversely across the troughlike member, a removable dam positioned within the molding form intermediate said wire positioning members and forming a closure for said opposite open end of the molding form, said dam having an opening therethrough in line with the wire positioning members, spring means secured to each of said pin-like projections, said first and second wire positioning members together forming means adapted to support a plurality of electrically conductive wires in spaced relation to one another, one end of each wire being attachable to an individual one of said spring means and tensionable thereby when each such wire is strung from one to the other of said wire positioning members through the opening of the dam.

References Cited in the file of this patent UNITED STATES PATENTS 1,502,343 MacPherson July 22, 1924 1,837,142 Bailey Dec. 15, 1931 2,215,066 Clark Sept. 17, 1940 2,334,534 Ballweg Nov. 16, 1943 2,402,338 Morehead June 18, 1946 2,685,107 Schultz Aug. 3, 1954 2,719,775 Erving Oct. 4, 1955 

